In order to meet an ever-increasing data recording density of an optical disk in recent years, the wavelength of a laser beam is becoming shorter and the NA of the objective lens is becoming higher for an optical head used to record and play back data. An optical head using the objective lens having a high NA, however, has a problem that it is quite sensitive to influences of spherical aberration caused by an error of thickness of the cover layer of the optical disk used as a recording medium.
To solve this problem, as is disclosed in Patent Document 1, an optical head provided with spherical aberration correcting means has been known. The optical head disclosed in Patent Document 1 performs electromagnetic driving by supporting a lens used to correct the spherical aberration on leaf springs.
Hereinafter, the configuration of the optical head will be described with reference to FIG. 20. As is shown in FIG. 20, the X axis is used for the optical axis direction. An aberration correction lens 41 is mounted on a lens holder 44, and a coil 42 is wound around the lens holder 44. A magnetic field is applied to the coil 42 by a magnet 43.
Leaf springs 45 are connected to an aberration correction base 46. Each blade spring 45 supports the lens holder 44 to be movable chiefly in the direction X. By providing two leaf springs 45, the aberration correction lens 41 is allowed to move in parallel easily in the X axis direction. Also, by making the blade plate 45 to be of a folding structure, it is possible to suppress a displacement of the aberration correction lens 41 in the Y axis direction caused by bending of the leaf springs 45. A position sensor 47 that detects the position of the aberration correction lens 41 in the optical axis direction is provided. In this example, the position sensor comprises an optical sensor.
When a specific DC current is supplied to the coil 42, the lens holder 44 receives thrust in the optical direction due to the function of the magnetic field induced by the magnet 43, and the leaf springs 45 bend accordingly. The aberration correction lens 41 then undergoes relative displacement with respect to the aberration correction base 46. In this instance, the aberration correction lens 41 stops and stands still at a position at which an elastic restoring force of the leaf springs 45 and the thrust that the coil 42 is receiving achieve equilibrium. The position sensor 47 generates a signal corresponding to the position of the aberration correction lens 41 in this instance, which enables position control to correct a position error from the target position by performing feedback control on a current value of the coil 42 as needed.
A flux of light having passed through the aberration correction lens 41 changes its divergent-convergence state with the position in the optical axis direction (the direction X), which gives rise to spherical aberration. Spherical aberration caused in this instance is the aberration inverse to the spherical aberration caused by a thickness error of the cover layer of the optical disk when the flux of light goes incident on the objective lens. The spherical aberration of a light spot irradiated onto the optical disk can be therefore corrected by the aberration correction lens 41.
Patent Document 1: Japanese Patent No. 3505525 (pp. 4-6, FIG. 4)